In an image-forming apparatus, such as a copying machine utilizing a so-called Carlson process, an electrophotographic photosensitive element is used. This element comprises a photosensitive layer on a base material which has an electric conductivity.
An electrophotographic photosensitive element repeatedly receives electric, optical, and mechanical shocks during the image-forming process. To protect the photosensitive element, a surface protective layer composed of a binder resin has been formed on the photosensitive layer thereof. This layer improves the durability of the photosensitive layer to these shocks.
A thermosetting silicone resin is generally used as the binder resin for improving the hardness of the surface protective layer. However, the use of the aforesaid heat-setting silicone resin presents the problem that the surface protective layer is brittle to sliding friction and is liable to be damaged. A variety of solutions have been attempted to try and avoid this problem.
One attempt was an electrophotographic photosensitive element which used a thermosetting silicone resin and a thermoplastic resin, such as polyvinyl acetate, as the binder resin for the surface protective layer. This type of protective layer is disclosed in JP-A-63-18354 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). An electrophoto-graphic photosensitive element which uses a thermosetting silicone resin and a butyl etherified melamine-formaldehyde resin as the binder resin is disclosed in JP-A-63-2071.
Also, an electrophotographic photosensitive element which uses a thermosetting silicone resin and an acrylic polymer as the binder resin is proposed in JP-A-60-3639.
However, when the thermosetting silicone resin and the thermoplastic resin are used as the binder resin for the surface protective layer, the sensitivity of the photosensitive element is insufficient. Another problem is found in the physical properties of the surface protective layer. The surface hardness of the combination binder resin is lower than the surface hardness of the thermosetting silicone binder resin alone. As a result, the surface protective layer is rather more likely to be damaged. In particular, the system using the thermosetting silicone resin and polyvinyl acetate has the problem that the coating composition for forming the surface protective layer lacks stability and when the coating composition is coated after the pot life, whitening occurs in the layer.
On the other hand, the binder resin made up of the thermosetting system and the butyletherified melamine-formaldehyde resin also has problems. The resins constituting the system are thermosetting resins and form a three dimensional structure having a high hardness after setting. Although the surface hardness of the surface protective layer becomes high, a large amount of voids are formed which become structural traps. These traps form between a silicone site and a melamine site in the protective layer owing to an insufficient compatibility between both of the sites. These traps result in the possibility of the binder resin having an adverse influence on the photosensitive characteristics of the electrophotographic photosensitive element. These adverse effects include the reduction of the charging characteristics, and lowering of the stability of the potential by repeated application of light exposure.
One attempt to avoid these problems was the use of a methyletherified melamine-formaldehyde resin in place of the butyletherified melamine-formaldehyde resin in the aforesaid system. The methyl etherified melamine-formaldehyde resin has a higher crosslinking property than the conventional butyletherified melamine-formaldehyde resin, and does not form a covalent bond with the Si--OH group of the thermosetting silicone resin during setting. Instead, it causes a sufficiently large molecular interaction with the Si--OH group of the thermosetting silicone resin, which improves the compatibility between the silicone site and the melamine site in the layer. This forms a compact layer having less structural traps. However, this system also has problems. When the methyl etherified melamine-formaldehyde resin is compounded with the thermosetting resin in an amount of over 15 parts by weight per 100 parts by weight of the non-volatile solid components of the latter resin in order to increase the electric conductivity of the layer using aromatic n electrons of melamine, a problem results. This problem is that the interaction between both of the resins is too strong which causes internal stress in the surface protective layer that forms cracks.
The above-described butyletherified melamine-formaldehyde resin does not have the strength interaction with the thermosetting silicone resin that the methyletherified melamine-aldehyde resin does. As a result, it was considered to use a combination of the butyletherified melamine-formaldehyde resin with the methyletherified melamine-formaldehyde resin. This combination could improve the electric conductivity of the layer by increases the number of aromatic .pi. electrons of melamine which were present. However, because both of the melamine-formaldehyde resins differed in setting or hardening temperature, a uniform layer could not be formed and there was the problem of cracks being formed.
The system of the thermosetting silicone resin and the acrylic copolymer is excellent in optical characteristics. The acrylic copolymer also has excellent compatibility with the thermosetting silicone resin compared to the use of polyvinyl acetate. The sensitivity characteristics of the coating are also improved compared to the aforesaid system using polyvinyl chloride. However, because the acrylic polymer which is used the aforesaid system has a high molecular weight between 8,000 and 60,000, the acrylic polymer is not easily dissolved in order to form a coating composition. Insufficient dissolution of the polymer in a coating composition creates additional problems. These problems include the inability to form a uniform layer, unevenness in the layer and white turbidity, of the layer. These defects reduce the transparency of the surface protective layer, which results in a deterioration of the sensitivity characteristics of the photosensitive element. They also may reduce the strength of the surface protective layer which results in the layer becoming brittle to sliding friction and susceptible to cracking.